Obesity and cancer – adding more weight to the evidence

A new study adds further evidence to the link between obesity and cancer

With the nation’s ever-expanding waistlines, obesity is never far from the headlines, and you may have seen that today is no exception. A new study – published in the Lancet – has analysed the link between body mass index (BMI) and the risk of 22 of the most common cancer types, and adds further weight to the evidence linking obesity and cancer.

Thanks to years of research, researchers were already pretty sure that seven different types of cancer – including oesophageal, bowel, pancreatic, gallbladder, breast, kidney and womb cancers – were more common among people who are overweight or obese. But the new study – which is of unprecedented size and rigour – brings more types of cancer into the picture

And although we don’t fully know why there’s a link, these new findings further reinforce the idea that being overweight or obese definitely does increase your risk of cancer.

So what did the new study show?

Weight linked to more cancers than ever before

The researchers, based at the London School of Hygiene & Tropical Medicine, looked at the anonymised medical records of 5.4 million people in the UK, where their GP had recorded their height and weight. This allowed the researchers to calculate each person’s BMI – a measurement used to see whether a person’s weight differs from what may be expected for their height.

The team then looked at whether a person’s BMI meant they were more likely to be diagnosed with cancer during the study.

It’s a huge study, and has analysed more data than any previous research. This is important as it gives us really strong evidence about how BMI affects cancer risk.

For the first time, BMI has been linked to 17 specific cancer types, 10 more than previously thought.

But the really interesting findings came when the team looked in detail at how risk changed as BMI increased.

A complex picture

These studies are complicated

So how did this break down by cancer type?

The results showed that obesity had the biggest effect on a woman’s risk of womb cancer – the higher a women’s BMI, the higher the chances of subsequently developing the disease.

They also found substantial effects on gallbladder, kidney and liver cancers.

And a high BMI was also linked to higher risks of colon, cervical, thyroid, ovarian and post-menopausal breast cancers, as well as leukaemia.

But it wasn’t just a high BMI where increased risks were observed. The team also found that being underweight was linked to a higher risk of mouth, stomach and lung cancers – but this could be explained by smoking as the effect disappeared among people who had never smoked, and these are all smoking-linked cancers.

They also found different-sized effects depending on whether you are male or female – being overweight increased the risk of liver cancer more for men than women, for example.

Overall, the team found a ‘Goldilocks’ effect – some cancers were more common in overweight people, others in those underweight. In other words, the healthiest to be, in terms of cancer risk, is not too large, not too skinny, but ‘just right’.

But when the researchers looked at how the risk of different cancers changed in people of increasing BMI, things started to get complicated.

For some tumour types, the risk of cancer increased gradually as BMI increased – a so-called ‘linear’ relationship. But for others, the risk changed unpredictably as BMI increased.

Data showing how cancer risk changed with increasing BMI – figure reproduce with permission from the journal (click to enlarge)

Ultimately, these results highlight how complicated the links between BMI and cancer are. Such studies will always act as a springboard for more research to understand these risks better, rather than giving a definite answer.

What this study doesn’t show

While important and vast in scale, we still don’t understand how and why BMI affects cancer risks. There are many theories, but no definite answer – perhaps because, as this study demonstrates, these risks are complicated and don’t always follow the same pattern.

To really answer the question, we need studies that get over a few hurdles that the current study didn’t address, for example:

Is BMI the best way of measuring obesity? Probably not. While it’s the best universal measurement we have it’s not clear if BMI is the most accurate way to measure obesity. In fact, a major limitation of this study was the lack of alternative measures for obesity (for example body fat percentage).

What about exercise? Physical activity can reduce the risk of many types of cancer, and wasn’t measured either, highlighting another gap in this research.

And perhaps most importantly for working out what advice to give the public, we still don’t know whether losing weight will help to lower cancer risk. To find this out, we’ll need really detailed data and studies with long follow-up times – but for now, all we can say is that keeping a healthy weight is important.

But despite these limitations the size and statistical robustness of this research makes it the most comprehensive analysis of the obesity/cancer relationship yet.

So what does this all mean for you?

Obesity is an important risk factor for cancer, and this study estimates that 12,000 cases of cancer a year in the UK are due to the nation’s weight problem, although previous research has put that number as high as 17,000. This is important, as studies like this can focus people’s minds on ways to prevent the disease.

But it’s not just about cancer: obesity is a risk factor for many conditions, including diabetes and heart disease. The evidence shows that the detrimental impact of obesity in the UK is mounting, and this study will support the political discussion around obesity at a national level.

It’s important to state, though, that research on this isn’t about blaming people for their cancer (something we’ve written about before), nor is it about researchers ‘telling people what to do’. It’s about trying to understand what cancer is, what causes it, and why – so researchers, doctors and policy makers can try to help minimise the terrible burden it places on our society.

And a final point – this new study also shines a light on the importance of anonymised data in medical research. The more data that’s made available for research, the more clearly researchers can identify the patterns and reasons for these risks – and ultimately work out how to minimise them.

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Cancer is caused by unexpressed emotions. The reason that scientists have found a link with obesity is because people eat a lot when emotional, to make them feel better. If we do not acknowledge, express or allow our emotions to flow through us then they become trapped inside us and as a result eat away at our physical bodies. It is really that simple. Cancer researchers, I ask you to research this cause. Thank you.

If studies suggest that some people might be genetically pre-disposed to being overweight then what’s to say that its not that same genetic pre-disposition that’s also linked to other medical conditions? If this is the case then, while it may still be possible to lose weight (and its obviously preferable for folks to maintain a healthy weight for a number of good reasons), losing weight may not actually afford any protection against other medical conditions that might be genetically unavoidable any way?

“And although we don’t fully know why there’s a link, these new findings further reinforce the idea that being overweight or obese definitely does increase your risk of cancer.”

Current research into hitherto unthought of influences on obesity (gut microbiome studies, genetic studies, etc etc etc) are showing that obesity is much more complex than the medical profession has assumed for many years. And cancer, too, is extremely complex. So it’s likely that this simple minded “obesity causes cancer” conclusion is at least wrong for many cancers, and may be wrong for all cancers. The same set of factors could be working with individual differences in people to cause both their cancer and their obesity.

Why is this so hard for medical researchers to grasp? We all know that correlation doesn’t mean causation, that has been repeated so often in recent years that ‘it’s a cliche – but then some group of researchers once again assumes that correlation is, in fact, causation – they just can’t figure out how!

It’s much smarter to acknowledge one’s ignorance, say “there is this link but we don’t know what it means” and continue research, trying to find the actual chains of biological mechanisms that lead up to different cancers. Then, we’ll know if obesity is a cause or a result of the process in some or all of the processes, and if it is central or peripheral.

Premature certainty stifles research. We’re many years away from coming to conclusions like this one, if we insist that such conclusions should be based on actual identified biological processes.

In so far as oesophageal cancer is concerned, the waistline size is probably a reflection of the upward pressure causing stomach acid to rise into the oesophagus. Persistent heartburn over a long period can lead to Barrett’s Oesophagus that carries a risk of dysplasia and oesophageal adenocarcinoma. Gastro Oesophageal Reflux Disease (GORD) is associated with this cancer, which therefore probably has a more direct and straightforward relationship to waistline size than some of the other cancers mentioned in the research. Excellent briefing note by CRUK!

I agree with Celia. The article doesn’t mention diet. Too much emphasis is put on exercise as a means of losing weight. While exercise is good for overall health and wellness, it is a poor way to lose weight.

Obesity is a by-product of our diet. 2 people can consume the same amount of calories and 1 could gain weight and the other 1 not because of their diet and predisposition for fat accumulation. Our diet affects our hormones and our hormones affects our fat cells.

Maybe, the foods that promote obesity are the same foods that promote cancer.

Observations have been made in societies that historically had a low rate of cancer when they started consuming a typical western diet(high in sugar and carbs) the cancer rate increased.

I wish all these smart people would start thinking about things differently and stop trying to show obesity as the cause of being sick.

Get a new hypothesis already. Maybe it’s hormones. Maybe what we eat affects the hormones. Sugar and carbs drive insulin drive fat. Maybe hormones are what regulates almost everything in our body and if our hormones are out of whack we have to regulate them – Either eat different and/or excercise or take medicine.

You simply cannot properly study obesity and any health outcome if you do not measure physical activity properly and take it into account in the analyses. Any study that doesn’t do this is ‘junk science’.

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